Level detector amplifier with controllable degenerative feedback

ABSTRACT

A fail-safe level detector comprising an AC amplifier circuit and a voltage breakdown device for controlling the amount of degenerative feedback so that sufficient gain for producing an AC output signal only occurs when a predetermined level of DC voltage causes the breakdown device to conduct and to assume its low dynamic impedance condition.

United States Patent [72] Inventor John 0. G. Darrow [56] ReferencesCited l N m p g UNITED STATES PATENTS [211 P 3,030,022 4/1962 Gittleman330/24x [221 FM Feb-2031968 3,106,684 10/1963 Luik 330/86X [451 i3,221,262 ll/1965 11611 1 330/24x [731 Cmlmy 3,281,639 10/1966 Potter eta1. 307/235x wlssvr e, a. a or oration of Pennsylvania Primary Examiner-Stanley T. Krawczewicz Attorneys-W. L. Stout and John B. Sotak [54]LEVEL DETECTOR AMPLIFIER WITH BLPiLnDEFENERATWE FEEDBACK ABSTRACT: Afail-safe level detector comprising an AC am- 8 plifier circuit and avoltage breakdown device for controlling [52] 0.8. CI. 307/235, theamount of degenerative feedback so that sufficient gain 246/187;330/24,330/86,330/1l0 for producing an AC output signal only occurswhen a [51] Int. Cl. H0311 5/20 predetermined level of DC voltage causesthe breakdown Field of Search 307/235; device to conduct and to assumeits low dynamic impedance 330/86, 110, 24; 246/167, 187(A) condition.

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LEVEL DETECTOR AMPLIFIER WITH CONTROLLABLE Y DEGENERATIV E FEEDBACK Myinvention relates to a fail-safe amplitude level detector and moreparticularly to a fail-safe circuit arrangement employing an ACamplifier and the voltage and dynamic impedance characteristics of abreakdown device for producing an AC output signal when and only whenthe amplitude of a DC input exceeds a predetermined value.

In various automatic control systems safety is of paramount importance.For example, in vehicle speed detection arrangements for mass and/orrapid transit operations, it is mandatory to determine the actual speedof. a moving vehicle and thereafter to compare the actual, speed withthe prescribed speed command request for a given area or section inorder to prevent injury to individuals and damage to equipment. That is,in such systems, it is a peremptory safety requirement that under nocircumstances should the actual speed of the moving vehicle exceed thepreselected speed command request for any given area. In one particulararrangement, the actual speed of a moving vehicle is derived by suitablespeed sensing apparatus, such as an axle driven frequency generatorwhich delivers AC signals having a frequency directly proportional tothe vehicular velocity. These AC signals obtained from the frequencygenerator are, in turn, applied to a suitable voltage limiter whichprevents an excess voltage swing .in either direction and provides thatthe signal amplitudes will be substantially constant. These limitedsignals are, in turn, applied to a suitable failsafe low-pass filterwhich is selectively chosen to have an upper frequency limitcorresponding to the speed command request for the particular area.Accordingly, the filter will only pass signals having frequencies belowthe upper frequency level. It will be appreciated that as the vehiclemoves from one area to the next, the upper frequency limit may beautomatically controlled by varying the filter components and theirvalues or by selecting one of a plurality of low-pass filter circuits inaccordance with the prescribed command requestfor each given area. TheAC output signals taken from the low-pass filter may, in turn, beconverted by a fail-safe rectifier to provide a DC output voltage whichis proportional thereto. Accordingly, if it is desired to insure that avehicle is proceeding at a speed below some preselected value, it ismerely necessary to measure the amplitude of the DC output signalssupplied by the rectifier. However, as in all vital portions of suchspeed command control systems, this measuring function must be performedby fail-safe apparatus which will not provide an output signal when thevehicle is moving in excess of the preselected command request. That is,it is of the utmost importance to exercise extreme care in designing andconstructing this portion of the apparatus in order to maintain thesecurity and integrity of the overall system. Accordingly, it is readilyevident that the detection apparatus must operate in a fail safe mannerso that any conceivable and foreseeable failure will result in acondition as least as restrictive and preferably more restrictive thanthat preceding the failure. For example, in such apparatus a circuitmalfunction or component failure should not be permitted to erroneouslysimulate or indicate a condition for holding or maintaining the vehiclesspeed, and normally, it is preferred that the failure should eitherprovide a warning such as flashing a light, sounding a buzzer, orinitiate a braking action for stopping the vehicle. Thus, in order toinsure the highest degree of safety to individuals as well as apparatus,it is necessary and essential that under no circumstances will a failurecause or be capable of simulating a true or valid speed indication.

Accordingly, it is an object of my invention to provide a new andimproved fail-safe circuit arrangement.

A further object of my invention is to provide a unique amplitude leveldetector circuit which operates in a fail-safe manner.

Another object of my invention is to provide an improved semiconductivecircuit arrangement which will provide an output signal when and onlywhen the amplitude of an input exceeds a predetermined value.

Yet another object of my invention is to provide a fail-safe leveldetector for measuring the amplitude of a DC input and only producing anAC output signal when the amplitude of the DC input exceeds apredetermined level.

Still another object of my invention is to provide a fail-safe circuitarrangement for providing an AC output signal when and only when theamplitude of the DC supply voltage exceeds the preselected value.

Still yet another object of my invention is to provide a failsafeamplitude level detector employing the dynamic impedance characteristicsof a breakdown device for controlling the gain of an AC amplifier.

Still yet a further object of my invention is to provide atransistorized level detector which operates in a fail-safe manner toproduce an output signal when and only when an input exceeds apredetermined level.

A still further object of my invention is to provide a feedback-type ofamplifier which is only capable of amplifying an AC input when a DCinput causes: a voltage device to break down and assume its low dynamicimpedance condition.

Yet still another object of my invention is to provide a failsafecircuit arrangement which is simple in construction, economical in cost,efficient and reliable in operation.

Briefly, the fail-safe level detector of the present invention employsan AC amplifier circuit and a voltage breakdown device. The amplifierincludes a transistor amplifying device having a degenerative feedbackcircuit. The degenerative feedback circuit is suitably interconnected bythe voltage breakdown device to the transistor for controlling theamount of feedback voltage and, in turn, the gain of the amplifier.Normally, the voltage breakdown device exhibits a high dynamic impedanceand only assumes a low dynamic impedance condition when a DC inputvoltage exceeds the breakdown voltage of the device. Accordingly, theamplifier will only produce an AC output signal when a DC input voltageexceeds a predetermined amplitude for causing the breakdown device toconduct and assume its low impedance condition so that the amplifiergain is increased due to the decrease in degenerative feedback.

The foregoing objects and other attendant features and advantages willbe more readily appreciated as the subject invention becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

The single FIG. is a schematic circuit diagram of a fail-safe leveldetector in accordance with the present invention.

As is well known, a feedback type of amplifier may be defined as anelectronic circuit in which a portion of the amplified output signal isfed back to the input terminals. When the feedback signal is out ofphase with the input and decreases the gain of the amplifier, it iscommonly referred to as negative feedback, degenerative feedback orsimply degeneration. While inadvertent feedback generally always occursin electronic circuits, the most common type of deliberate currentcontrolled feedback, in transistor circuits, is emitter degeneration. Aswill be described in greater detail hereinafter, by controlling theimpedance characteristic of the emitter electrode connection, the amountof degenerative feedback and, in turn, the given losses therein may beemployed to control the amplification qualities of the transistoramplifier. That is, a high value of emitter impedance willproportionally increase the negative or degenerative feedback whichtherefore materially reduces the gain of the amplifier therebyeffectively reducing the output of the amplifier. The present inventionmakes use of this operating principle in a unique manner wherein thecondition of an amplifier is effectively controlled and an AC outputsignal is only available when the magnitude of a DC input exceeds apredetermined value.

Referring to the single FIG. of the drawing, there is shown a preferredembodiment of the electronic amplitude level detector of the presentinvention. As previously mentioned, the amplitude voltage level detectoroperates in a fail-safe manner to measure the amplitude of a DC inputvoltage which is representative of the actual speed of the movingvehicle in an automatic speed control system. That is, the DC inputvoltage supplied by the speed sensing apparatus is inverselyproportional to vehicle speed and is shown applied to the inputterminals 1 and 2. As shown, a current limiting resistor. R1 has one ofits ends connected to the input terminal 2 and the other end connectedto the cathode of the zener diode Z1 the anode of the zener diode Z1 isconnected to the other input terminal 1 which is appropriately groundedthereby forming a common terminal. The resistor R1 and zener diode Z1form a voltage regulator which stabilizes and provides a substantiallyconstant operating and biasing supply voltage for improved operation. Inaddition to its voltage regulating feature, the voltage breakdown deviceor zener diode also exhibits the unique characteristics of having a highdynamic impedance condition when not conducting and of having a lowdynamic impedance condition when rendered conductive and operatingproperly. As will be described in greater detail hereinafter, thisunique characteristic is employed in the present invention for insuringthat the level detector operates in a fail-safe manner.

A voltage dividing network consisting of series connected resistors R2and R3 is connected in parallel across the zener diode Z1. The resistivevalues of the voltage dividing network are selected to provide theproper forward biasing potentials for the base-emitter electrodes of theNPN transistor Q1 which forms the amplifying or active element of theamplifier circuit. The transistor 01 includes a common emitter electrode6, an output collector electrode 7 and an input base electrode 8. Asshown, base electrode 8 is directly connected to the junction point J1of the voltage dividing network comprising resistors R2 and R3. Acapacitor C1 is connected between the junction point J1 for blocking theDC voltage from the signal source and for coupling the input AC signalsto the base electrode 8.

- As shown, an AC input signal, which may be derived from any convenientsignal source, such as a suitable sinusoidal oscillator or pulsegenerator, is applied across input terminal 3 and common terminal 1. Thecollector electrode 7 is connected to the upper portion of the voltagedivider network and, in turn, to the cathode of the zener diode Z1 bycollector load resistor 4. The emitter electrode 6 is connected throughresistors R5 and R6 to the common grounded terminal 1. While it is wellknown that the external emitter swamping resistors R5 and R6 temperaturestabilize the biasing conditions for improving amplifier operation, itis also evident that the resistors R5 and R6 result in seriesdegeneration which causes an.increase in input resistance and a decreasein the gain of the amplifier. Generally, the latter conditions may beresolved by. simply placing a suitable bypass impedance, such as acapacitor, in parallel with the emitter resistance to provide a lowimpedance path for the AC signal. Accordingly, a bypass capacitor C2 isshown connected from the junction point J2 resistors R5 and R6 to thecathode of the zener diode Z1 It has been found that by uniquelyemploying this principle of operation, the amount of negative feedbackand, in turn, the gain of an amplifier may be effectively controlled byvarying the impedance of the capacitive bypass circuit. That is,sincemaximum degeneration occurs when the emitter resistance is not bypassedand since minimum degeneration occurs when the emitter resistance isbypassed by through capacitor C2, the amplifier gain and, in turn,amplifying characteristics may be I effectively controlled. Therefore,by suitable proportioning resistors R4, R5 and R6 so that the effectivebase-to-emitter voltage is substantially equal to the AC input signalwhen the resistor R6 is bypassed and so that the AC base-to-emittersignal is effectively zero when the resistor R6 is unbypassed, thetransistor amplifier may either have a relatively high gain or have acomparatively low gain, respectively. For example, a R4 As shown. thecollector electrode 7 capacitor C4 for isolation purposes. It will beappreciated, that the AC output developed across output terminals 4 and1 may be applied to any suitable utilization device, such as a vitaltype of underspeed relay after being appropriately amplified andrectified, the. purpose of which will be described hereinafter.

Turning now to the operation, it will be initially assumed that themoving vehicle is proceeding at or below the preselected speed commandrequest so that the DC signals generated by the speed sensing apparatusand applied to the DC input terminals 1 and 2 are above a preselected orpredetermined level. As previously mentioned, the speed sensingapparatus is only capable of generating a sufficient level of DC voltagewhen the vehicle is proceeding at a speed lower than a preselected speedcommand request for the particular area. The voltage breakdowncharacteristic of the zener diode Z1 is appropriately selected torequire a potential level substantially equal to the voltage level ofthe sensing apparatus when the vehicle is moving below the preselectedspeed command request. Accordingly, under this condition the DC inputvoltage is of sufficient magnitude to cause the Zener diode Z1 tobreakdown thereby causing the diode to conduct and exhibit a low dynamicimpedance. Since the voltage across the zener diode remainssubstantially constant over a wide range of voltage and current changes,the various biasing voltages supplied to transistor amplifier insurestable operation. With the zener diode conducting and exhibiting a lowimpedance the resistor R6 becomes bypassed by capacitor C2 so that onlya prescribed amount of negative feedback voltage developed acrossresistor R5 is present. The purpose of this limited amount of feedbackinsures that full gain of the amplifier is of a controllable amount.Since a low impedance path now extends from the junction point J2,through bypass capacitor C2, through the zener diode Z1 to the commonterminal 1, little, if any, negative feedback voltage is developedacross resistor R6. Accordingly, the AC input signals applied to inputterminals 3 and 1 are amplified, and an AC output signal is providedacross the output terminals 4 and 1. It will be appreciated that theamount of AC outputpower which is .available at the collector electrode7 is a function of the amplifier gain minus the feedback power. Aspreviously mentioned, this AC output signal on terminals 4 and 1 aftersuitable amplification and rectification may, in turn, be employed toenergize an underspeed relay thereby indicating that the speed of thevehicle is not in excess of the preselected command request. That is,the presence of an AC output signal of a given amplitude at theterminals 4 and 1 may be construedas a true or valid indication that thevehicle is proceeding at or below the preselected speed command request.

Let us now assume that the actual speed of the vehicle increases to apoint beyond the predetermined speed command request so that the circuitoperation may be analyzed under this condition. Under this condition,the axle generator now produces a signal of increased frequency. Thesehigher frequency signals are greatly attenuated due to the inherentrejection characteristics of the low-pass filter, so that the DC outputproduced by the rectifier network is proportionally reduced at thistime. Accordingly, the level of DC input voltage applied to terminals 2and 1 is substantially below the zener threshold or breakdown voltage ofthe diode Z1. Now with an insufficient magnitude of DC voltage appliedacross the zener diode Z1, the diode will not conduct and will exhibit ahigh dynamic impedance. In view of the high impedance condition of thezener diode Z1, the capacitive bypass circuit extending from thejunction point J2 of the resistors R5 and R6 to the common terminal 1will appear as an open circuit and, therefore, an appreciable amount ofnegative feedback voltage will be developed across resistor 6. Inpractice, the resistive value of resistor R6 is chosen to be relativelyhigh so that the gain of the amplifier is proportionally reduced. Thatis, since the negative feedback voltage now developed across resistorsR5 and R6 is greatly increased, the effective AC base-to-emitter voltageis substantially zero. Accordingly,

As previously mentioned, the amplitude level detector must operate in afail-safe manner so that no conceivable component or circuit failurewill be capable of producing an AC.

output signal on the output terminals 4 and lat any time. It has beenfound advantageous to chose a relatively high value of resistance forresistor R1 in order to insure that a capacitive bypass circuit cannotbe established from junction point J2, through capacitor C2 and, inturn, through resistor R1 to terminal 4 and through the DC input sourceto terminal 1. Further, it will be noted that if the zener diode becomesshort circuited, the necessary biasing or supply voltages are notavailable and, therefore, the required amplifying characteristic of thetransistor are not functional. If the zener diode becomes opencircuited, it is quite apparent that the required low impedance bypasspath between the junction point J2 and common terminal 1 is not presentso that maximum degeneration occurs and no appreciable AC voltage isagain available at the output terminals 4 and '1. If the zener diodebecomes leaky and conducts at some voltage lower than its normalbreakdown voltage, the dynamic impedance exhibited by the diode is stillgenerally sufficient to, cause an appreciable amount of degeneration toovercome the gain of the amplifier and, therefore, the amount of ACvoltage available across ter minals 4 and 1 is insufficient to causeenergization ofthe relay. An open-circuit failure of the currentlimiting resistor R1 is obviously a safe condition. Normally,fail-safeness is based on the premise that resistors or resistiveelements cannot become short circuited due to the particular type ofresistors, namely, carbon-composition, employed'in circuits which mustoperate 'in a fail-safe manner. It will be noted that the various othercomponents and elements constituting the amplifier circuit will eitherfail in a safe manner or destroy the circuit integrity to the pointwhere an AC output signal is not produced.

Therefore, it will be observed that the presently described leveldetector operates in a fail-safe manner so that an AC output signal isavailable at the outputterminals 4 and 1 when and only when apredetermined value of DCinput is applied to the input terminals 2 andl.

Further, it may be mentioned that the circuit parameters of thedescribed detector have been selected to preferably employ a zener diodehaving a threshold" or breakdown voltage in the range of 6 to 8 volts inthat no presently known diode in this range can conduct at a lower thanrated zener voltage and yet exhibit a low dynamic impedance. However, itis readily understood that zener diodes having other voltage zenerratings may equally well be employed when his possible to positivelyinsure that when the diode breaks down at a lower than its rated zenervoltage, it will not exhibit a low impedance condition.

Also, it will be appreciated that while the present invention has beendescribed in terms of zener diodes and transistor amplifiers, it isreadily understood that other voltage stabilizing or breakdown devicessuch as man glow or gas regulator tubes and that other amplifyingdevices such as gas or vacuum tubes may be employed with equal success.

Although a common-emitter configuration and an NPN transistor have beenillustrated, it is understood that commoncollector or common-baseconfigurations as well as transistors of opposite conductivity, that is,PNP transistors maybe used in practicing the present invention by merelyreversing the polarity of the direct current input and of the zenerdiodeas is well known.

In addition, it will be appreciated that while an emitter degenerationamplifier circuit has been illustrated in practicing the presentinvention, it is readily understood that various other types of feedbackamplifiers employing series or shunt degeneration circuits may beequally used in practicing the subject invention,

Further, it wil also be appreciated that while this invention findsparticular iutility in speed control systems, it is readily evident thatthe invention is not merely limited thereto but may be employed invarious other systems and apparatus which require the security andsafety inherent in the inven tion. But regardless of the manher in whichthe invention is used, it is understood that various alterations may bemade by persons skilled in the art without departing from the spirit andscope of this invention. It will also be apparent that othermodifications and changes can be made in the presently describedinvention, and, therefore, it is understood that all changes andequivalents and modifications within the spirit and scope of thisinvention are herein meant to be included in the appended claims. Ilclaim: i l. A fail-safe level detector for producing an output signalwhenand only when an input exceeds a predetermined value comprising afirst amplifying means having degenerative feed back and a second meanselectrically connected to said first means for controlling the biasingsupply voltage and said degenerative feedback whereby said firstamplifier means is only capable of producing; said outputwhen said inputexceeds said predetermined value.

2. A fail-safe level detector as defined in claim 1, wherein said firstmeans comprises a feedback type of amplifier.

3. A fail-safe level detector as defined in claim 1, wherein said firstmeans comprises a semiconductive amplifier circuit,

4. A fail-safe level detector as defined in claim 3, wherein saidamplifier circuit includes a solid state device having a swampingresistor.

5. A fail-safe level detector as defined in claim 3, wherein saidamplifier circuit includes a transistor having an emitter. loadresistor.

6. A fail-safe level detector as defined in claim 3, wherein saidamplifier circuit comprises a common-emitter configura- 3 tion.

7. A fail-safe level detector as defined in claim 3, wherein saidamplifier circuit includes a controllable series degeneration circuit.

8. A fail-safe level detector as defined in claim 1, when said secondmeans includes a voltage breakdown device.

9. A fail-safe level detector as defined in claim 1, wherein said secondmeans comprises a voltage responsive device having a relatively highimpedance value when nonconducting and a relatively low impedance valuewhen conducting.

10. A fail-safe level detector as defined in claim 8, when said voltagebreakdown device comprises a zener diode.

11. A fail-safe circuit arrangement comprising, a feedback type oftransistor amplifier, and a voltage breakdown device electricallycoupled to said transistor amplifier for controlling the biasing supplyvoltage as well as the amount of feedback and, in turn, the gain of saidtransistor amplifier whereby an output signal is produced when and onlywhen said breakdown device becomes conductive and assumes a lowimpedance condition.

'12. A fail-safe circuit arrangement as defined in claim 11, whereinsaid amplifier includes a series degeneration circuit.

13. A fail-safe circuit arrangement as defined in claim 11, wherein saidamplifier comprises an emitter swamping resistor and a controllablebypass capacitor. i

14. A fail-safe circuit arrangement as defined in claim 1;], whereinsaid voltage breakdown device comprises ,a zener diode. l 15. Afail-safe circuit arrangement as defined in claim 1-1,? wherein saidtransistor amplifier comprises a common-emitter configuration.

16. A fail-safe circuit arrangement comprising, a DC input,

. an AC input, an AC output and a common terminal, ,a series' connectedcurrent limiting resistor and zener diode electriconnected between thejunction point of said fourth and fifth resistors and the junction ofsaid current limiting resistor and zener diode, a coupling capacitorconnected between the junction point of said first and said secondresistors and the AC input terminal whereby AC output signals arepresent on said AC output terminal when and only when a DC voltage ofsufficient amplitude is present on said DC input terminal to cause saidzener diode to conduct and assume a low dynamic impedance condition.

1. A fail-safe level detector for producing an output signal when andonly when an input exceeds a predetermined value comprising a firstamplifying means having degenerative feedback and a second meanselectrically connected to said first means for controlling the biasingsupply voltage and said degenerative feedback whereby said firstamPlifier means is only capable of producing said output when said inputexceeds said predetermined value.
 2. A fail-safe level detector asdefined in claim 1, wherein said first means comprises a feedback typeof amplifier.
 3. A fail-safe level detector as defined in claim 1,wherein said first means comprises a semiconductive amplifier circuit.4. A fail-safe level detector as defined in claim 3, wherein saidamplifier circuit includes a solid state device having a swampingresistor.
 5. A fail-safe level detector as defined in claim 3, whereinsaid amplifier circuit includes a transistor having an emitter loadresistor.
 6. A fail-safe level detector as defined in claim 3, whereinsaid amplifier circuit comprises a common-emitter configuration.
 7. Afail-safe level detector as defined in claim 3, wherein said amplifiercircuit includes a controllable series degeneration circuit.
 8. Afail-safe level detector as defined in claim 1, when said second meansincludes a voltage breakdown device.
 9. A fail-safe level detector asdefined in claim 1, wherein said second means comprises a voltageresponsive device having a relatively high impedance value whennonconducting and a relatively low impedance value when conducting. 10.A fail-safe level detector as defined in claim 8, when said voltagebreakdown device comprises a zener diode.
 11. A fail-safe circuitarrangement comprising, a feedback type of transistor amplifier, and avoltage breakdown device electrically coupled to said transistoramplifier for controlling the biasing supply voltage as well as theamount of feedback and, in turn, the gain of said transistor amplifierwhereby an output signal is produced when and only when said breakdowndevice becomes conductive and assumes a low impedance condition.
 12. Afail-safe circuit arrangement as defined in claim 11, wherein saidamplifier includes a series degeneration circuit.
 13. A fail-safecircuit arrangement as defined in claim 11, wherein said amplifiercomprises an emitter swamping resistor and a controllable bypasscapacitor.
 14. A fail-safe circuit arrangement as defined in claim 11,wherein said voltage breakdown device comprises a zener diode.
 15. Afail-safe circuit arrangement as defined in claim 11, wherein saidtransistor amplifier comprises a common-emitter configuration.
 16. Afail-safe circuit arrangement comprising, a DC input, an AC input, an ACoutput and a common terminal, a series connected current limitingresistor and zener diode electrically connected between said DC inputand said common terminals, a voltage dividing network including a firstand a second connected across said zener diode, a transistor having anemitter, a collector and a base electrode, the base electrode connectedto the junction point of said first and said second resistors of saidvoltage dividing network, a third resistor connecting the collectorelectrode to the junction of said series connected current limitingresistor and zener diode, a series connected fourth and fifth resistorsconnecting the emitter electrode to said common terminal, a bypassingcapacitor connected between the junction point of said fourth and fifthresistors and the junction of said current limiting resistor and zenerdiode, a coupling capacitor connected between the junction point of saidfirst and said second resistors and the AC input terminal whereby ACoutput signals are present on said AC output terminal when and only whena DC voltage of sufficient amplitude is present on said DC inputterminal to cause said zener diode to conduct and assume a low dynamicimpedance condition.